1. Field of the Invention
The present invention relates to a high-power optical signal generator including at least a laser as the signal source and at least an optical amplifier operatively connected to the laser. The invention is particularly suitable for telecommunication equipment, such as cable television. (CATV) and signal distribution networks.
2. Background and Objects of the Invention
At present, high-power and high-speed transmissions are limited by the maximum power of single-frequency semiconductor lasers of the DFB (distributed feed-back) type used as signal sources. In particular, it is difficult to realize semiconductor lasers which have an output above 2-4 mW and which are reliable. Furthermore, if bias current is commutated or modulated at a high modulation speed in such lasers, the fast current changes cause a change in the frequency of the light emitted by the device. When the signal then propagates in a dispersive means (such as an optical fiber), these frequency changes turn into propagation time variations and the quality of the received signal is degraded.
If the laser operates with continuous wave (CW) and an external width modulator is positioned downstream, however, the frequency noise problem is eliminated. Nevertheless, the width modulator, generally an integrated optics passive device, causes an additional loss due to the coupling losses which take place owing to light transition from an optical fiber to a wave guide of the modulator and vice versa.
These losses involve a power penalization of 3-4 dB, further reducing the available optical power.
The use of an amplifier placed downstream of the modulator in order to increase output power to 20-40 mW is described in PROCEEDINGS ECOC '91, (Post-Deadline Paper), pages 72-75, from P.M. Gabla et al.
In this document an experiment is described using a DFB laser transmitter connected to a Mach-Zender external modulator and an erbium-doped optical fiber postamplifier (EDFA) in order to raise the signal level up to +12 dBm. However, the transmission path of the described device goes through 26 sections, each one including one erbium doped optical fiber amplifier (EDFA) and one fiber coil.
Also, the DFB laser used in the experiment had emission at a wavelength of 1553 nm, while the maximum absorption of the amplifier fiber was at a wavelength of 1533 nm.
Furthermore, in OFC '92, pages 242, 243 (Y.K. Park et al.) a long distance transmission experiment is described in which a DFB laser at 1558 nm wavelength emission was connected to a Mach-Zender external modulator and then to a power amplifier. The described device had a series of two connected erbium doped optical fiber amplifiers (EDFA), each one of them with bidirectional pumping.
In order to obtain the better performances of the amplification stage connected to the laser, and of the line amplifiers, the emission wavelength of the continuous wave laser must be fitted as much as possible to the gain peak wavelength of the amplifier, which, if carried out in a fiber made of silica doped with erbium, has wavelength values of about 1531 or 1536 nm, which is related to the dopant, e.g., germanium or alumina, which is used to modify the refraction index of the fiber core.
In order to satisfy this requirement, it is necessary to use a DBF laser at a selected wavelength but this involves, among other things, manufacturing difficulties and a significant increase in cost.
The known structure also has drawbacks, due, among other things, to the requirement of limiting the noise generated by the amplifier, which is particularly critical for some applications and which requires the maintenance of a high pumping power level along the whole fiber. This condition requires, on the one hand, feeding high pumping power to the fiber, thus reducing the amplifier efficiency, and on the other hand, having high power pumping lasers which have a reduced reliability.
In order to supply such high pumping power, pumping splitting and multi-stage amplifications may also be required, which makes the structure more complex and affects the whole efficiency: for instance, Y. Park et al. in OFC '92 describe the use, as a power amplification unit, of two fiber amplifiers, each of them pumped with two pumping lasers having powers respectively of 15.3 dBm and of 17.3 dBm, for an output power of 16 dBm.
There are also known laser fibers, doped with erbium, as described in PROCEEDINGS ECOC '91, pages 149-152 by G. Grasso et al. Such lasers, nevertheless, require the use of a diode pumping laser, which is commercially available with emission powers which are not optimal for fiber laser operation, so that the fiber laser has limited efficiency, particularly for the aforesaid uses in telecommunications.
It has been found that the combination of a fiber laser and a fiber amplifier, in which the pumping power is shared between the laser and the amplifier, allows the achievement of high efficiency and low noise generation, overcoming the typical limits of the known solutions and of the single components.
An object of the present invention is, therefore, to provide a high power optical signal generator, particularly for telecommunications use, which has high efficiency, reliability and low cost.